A standard assembly machine, for instance serving to put together small- to medium-sized articles such as hinges or lamp sockets, sits adjacent a conveyor or transfer machine that brings to it a succession of partially constructed and/or assembled workpieces that the assembly machine must perform some operation on. Normally the assembly machine is provided with one or more grippers and/or with a tool that performs some desired operation on the workpiece, often adding to the workpiece a part that is fed in by another conveyor.
In a standard system described in German patent 4,320,501 of G. Dornieden each workpiece is carried on a respective holder that is moved along by a special-duty conveyor through a plurality of work stations to some of which are delivered parts that are added to the workpiece. Immediately downstream of the farthest downstream work station is an emptying station that either strips each workpiece off its holder so that the holder can be recirculated to a point upstream of the farthest upstream work station and fitted with a new unfinished workpiece or that actually moves each holder and its workpiece along to another machine or set of machines. The conveyor is a worm extending the full length of the production line from the farthest upstream station to the farthest downstream station and formed with a continuous screwthread having pitched sections extending helically of the worm's axis alternating with unpitched sections lying in respective planes perpendicular to this axis. Each holder has formations complementarily engaged with this screwthread so that as the worm rotates the holders are advanced in steps, moving axially downstream when their formations engage the pitched screwthread sections and stationary while engaging the unpitched sections.
In commonly owned and copending application Ser. No. 09/022,143
filed Feb. 11, 1998 an assembly machine is described having a frame defining at least one work station, a transfer mechanism for feeding a succession of workpieces one at a time through the station, and at least one tool at the station movable between an operative position engageable with the workpiece in the station and a retracted position out of engagement with the workpiece in the station. A push-pull actuator has an outer end connected to the tool and an inner end connected to a cam carried on an output of a drive mounted on the frame for displacing the tool between its positions synchronously with feeding of the workpieces through the station. A pneumatic spring mounted on the frame is braced against the tool and has a compartment pressurizable to urge the tool into the operative position. The back compartment is continuously pressurized with a gas under a generally constant pressure.
These systems, which both use the cam-type shaft-driven conveyor, have several disadvantages. One cannot pick one holder and workpiece out of the system, for instance because it is defective or to perform a test, without creating a gap that will cause the devices at the stations to malfunction or report a problem and shut down the system. Similarly one cannot just insert a workpiece and its holder into the row without disrupting production. Furthermore the cam-drive system is expensive and complex, and expanding the system to extend its conveyor through farther stations is very complex and often prohibitively expensive.